Multi-Dimensional Flow Characteristics of Bubbly Flows.

(1)研制了一种利用超声脉冲多普勒效应和数据处理的测量系统。所提出的测量系统可以测量通道内气泡流的瞬时混合速度分布。通过对实测的瞬时混合速度分布进行统计处理，可以得到两相的速度分布、通道内液相的空隙率分布和湍流强度以及气泡周围的液体速度场。(2)充分发育气泡流的流动结构受液体连续相流型的支配，该流型比液体单相流型平坦。由于气泡由于浮力和界面剪切应力的差异而上升，因此两相之间的相对速度在通道中几乎是恒定的。(3)除槽壁上的空隙外，槽内空隙率基本不变。(4)气泡流湍流度的两相乘数定义为气泡流与水单相流的湍流度之比。气泡的存在既有促进湍流的作用，也有平抑湍流的作用。当孔隙率小于0.3%时，冷静效果优于促进效果。在孔隙率大于0.5%的情况下，随着孔隙率的增加，促进作用越来越大。(5)气泡周围的液体流场可分为三个区域；一个边界层，一个过渡区和一个主流区。边界层强烈依赖气泡上升运动，流动不受空气和水流速变化的影响。过渡区受气泡运动和主液流的共同影响。由于这两种影响的混合存在，该地区的速度波动很大。主要流动区域远离气泡，气泡运动不影响流动。(6)边界层厚度与气泡直径关系密切，与气泡间距关系不大，并提出了边界层厚度与气泡直径的经验相关关系。给出了气泡表面到该区域的距离随气泡雷诺数和平均空隙率的变化规律及其经验相关性。少

(1) A measurement system using Doppler effect of ultrasonic pulse and data processing has been developed. The proposed measurement system can measure instantaneous mixture velocity profiles of bubbly flows in the channel. By treating statistically the measured instantaneous mixture velocity profiles, the velocity profiles of both phases, the void fraction profile and turbulence intensity of the liquid phase in the channel and the liquid velocity field surrounding a bubble can be obtained.(2) Flow structure in fully developed bubbly flows is governed by the flow profile of a liquid continuous phase which is flatter than that of liquid single phase flows. Since bubbles go up due to the difference between buoyancy and interfacial shear stress, the relative velocity between both phases are almost constant in the channel.(3) Void fractions in the channel are almost constant except for those near the wall.(4) The two-phase multiplier of turbulence in bubbly flows is defined as a ratio of tur … More bulence intensity in bubbly flows to that in water single phase flows. The exist of bubbles has both effects of turbulence promotion and dispassion. For void fraction less than 0.3%, the dispassion effect is superior than the promotion effect. In the case of void fraction larger than 0.5%, the promotion effects becomes greater with an increase in void fraction.(5) The liquid flow field surrounding bubbles can be classified into three regions ; a boundary layer, a transition region and a main flow region. The boundary layer is strongly dependent on the bubble rising motion and the flow is not influenced with changes in air and water flow rates. The transition regions influenced by both effect of the bubble motion and the main liquid flow. Velocity fluctuation is high in this region because the mixing of these two effects exists. The main flow region is located far from bubbles and the bubble motion does not affect the flow.(6) The boundary layer thickness is dependent strongly on the bubble diameter and slightly on the distance between bubbles and its empirical correlation is proposed. The distance from the bubble surface to this region changes according to both bubble Reynolds number and the average void fraction and its empirical correlation is also proposed. Less